The present invention relates to ultrasonic flow metering. Ultrasonic flow metering, or the determination, using ultrasonic energy, of flow velocity through a conduit, such as a pipeline, is based on determining the effect of fluid flow on the upstream versus downstream transmission time of an ultrasonic signal passing diagonally through the fluid in the pipeline. Ultrasonic flow metering is particularly advantageous because it can be performed non-intrusively, i.e., without requiring intrusion into the pipeline. The ultrasonic flow meter transmitting and receiving transducers are suitably clamped on to the pipeline and ultrasonic signals are injected through the pipewall. The flow rate is determined by measuring the difference in transit time between upstream and downstream ultrasonic signals. Since the effect of flow on transit-time, even at high flow velocities, is small, it is essential to avoid even small uncertainties in this measurement. Unfortunately, the ultrasonic signal which is used for detection is of low resolution relative to the needed time detection resolution, since it generally consists of waves of periods of from hundreds to thousands of nanoseconds, while the needed detection resolution is of the order of from picoseconds to only a few nanoseconds at best.
In addition, to detect the arrival time of the sonic signal implies that there is a detectable "beginning" of the signal. Unfortunately, the signal does not arrive with a sharp front edge, but rather is a relatively slow buildup of a basically sinusoidal waveshape, due to the high "Q" of the ultrasonic transmitter, as well as the sonic resonance of the pipe wall or other structure through which the wave must pass in order to enter the liquid stream. These metallic structures are generally highly resonant, and contribute to the slow buildup of the waveshape. Thus at best, the beginning of the signal is hard to detect, and an error in detecting the arrival by even only one cycle of the receive signal frequency has a disastrous effect on the measured flow due to this effect on the apparent upstream versus downstream detection time difference.
Furthermore, even if the first arrival cycle is robust, it must be recognized that there is a background level of noise, much of it at the frequency of the receive signal itself, and therefore unfilterable. Accordingly, in present systems, detection of the actual arrival time of the beginning of the receive signal is either very difficult, uncertain as to its arrival time, or simply impossible in any real way, depending on the actual character of the signal and the ambient noise present when it arrives.